Method of using explosives to coat a metal body



6, 1968 M. DAVIS ETAL 3,395,444

FIG 2 United States Patent 3,395,444 METHOD OF USING EXPLOSIVES TO COAT A METAL BODY Myron Davis, Schenectady, N.Y., and Ronald J. Carlson,

Galloway, Ohio, assignors to General Electric Company, a corporation of New York Filed Aug. 1, 1966, Ser. No. 569,286 8 Claims. (Cl. 29529) This invention relates to a process for coating base metals with a thin layer of a coating metal. More particularly, the invention relates to a process for coating base metals in which the surface of the base metal is deformed and the coating metal tenaciously coated thereon.

It is well known to coat base metals with another relatively thinner layer of a coating metal for various purposes. For example, in the manufacture of bearings, such as those having a bearing surface as a Babbitt metal and the like, the base metal such as a ferrous material is first tinned or coated with a relatively thin layer of meta-1, such as tin, which adheres to the base metal and at the same time forms a compatible base for the Babbitt metal. A common practice in tinning such bearings is, after cleaning and fluxing, to dip the entire bearing surface in molten tin, masking those parts which are not to be coated. The Babbitt metal is then typically placed on the tinned surface as by centrifugal casting. In some cases the base metal is roughened or dovetailed to increase the adherence of the tin to the surface. With the ever increasing trend to larger bearings and surfaces to be coated with such metal, the need has arisen for a more convenient method of preparing such bearings without the use of relatively large baths of molten tinning material. It is also inconvenient and time consuming to dovetail the surface of the base to be coated with the tinning metal.

A primary object of the present invention, therefore, is to provide a method for tinning or forming layers of coating metals on base metal which is readily carried out with a minimum of coating metal and which forms a tenacious base for bearing metals or other metal.

Briefly, the present invention relates to a method for coating base metal with a relatively thin layer of coating metal which comprises placing a layer of the coating metal in closely spaced relationship to the surface of the base metal to be coated and separated therefrom by an apertured structure such as a metal screen and placing a layer of detonating explosive over the coating metal layer and detonating the explosive layer. The explosive force of the detonating explosive deforms the surface of the base metal, the coating metal being converted to the liquid phase and being extruded between the interstices of the screen, assuming the roughened contour of the deformed surface of the base metal and providing a base for bearing metal or other metal which is placed thereon.

This invention is to be distinguished from explosive welding as it is normally understood, such explosive welding involving a solid state phenomenon to achieve bonding. On the other hand, in the present invention sufficient internal energy is imparted by the chemical explosive energy to the coating metal to render the coating metal molten so that it actually wets the base metal. This melting of the coating metal, combined with high collision velocity, eliminates the need for a flux since surface films on the base metal or metal to be coated are penetrated or driven off. Generally speaking, the invention is particularly applicable to materials having relatively low melting points and low heats of fusion energy above those of tin. Among such metals in addition to tin are lead, bismuth, selenium and thallium.

Those features of the invention which are believed to be patentable are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further advantages and objects thereof appreciated from a consideration of the following description and the drawing in which FIGURE 1 shows a portion of a base metal object with the coating arrangement in place. FIGURE 2 is a cross-sectional view of the base metal surface with the coating structure in place, FIGURE 3 shows a portion of the coated structure obtained, and FIGURE 4 is a cross-sectional view of the structure with an overlayer of bearing or other metal on the explosively coated layer.

The use of detonating type explosives in forming metals and compacting powdered materials, such as metals, is well known. However, to our knowledge, this technology has never been applied to the coating of a base metal in which the surface of the base metal is deformed in the present manner with the coating metal fusing and conforming to the deformations and affording a roughened surface having reentrant portions to serve as an adherent compatible base for other metals, such as bearing metals and the like.

The explosives which are useful in the present connection are, as pointed out above, of the detonating type which has high brisance or generates high peak pressures with low energy and is to be distinguished from the defiagrating type of explosive which generates high energy and relatively low pressure. Generally speaking, by such detonating explosives are meant those which have a velocity of detonation of more than about 1200 meters per second in air and up to the sonic velocity of the metal in the combination with the highest sonic velocity. Various detonating explosives which fit this description will occur to those skilled in the art. Among such materials are pentaerythritoltetranitrate known as PETN. The deonating explosive is normally in the form of thin sheet material and typically contains by weight parts of PETN, 7.5 parts of butyl rubber and 7.5 parts of a thermoplastic terpene resin known as Piccolyte S10 manufactured by the Pennsylvania Industrial Chemical Corp. This explosive material is available in strong, flexible sheets of various thicknesses and the composition has a velocity of detonation of approximately 7000 meters per second. Other useful detonating explosive materials will occur to those skilled in the art including those described in US. Patents 3,043,521 and 3,205,574.

Any of the metals which have a low melting point and low heat of fusion energy such as tin and the like can be used as the coating material. It has been found necessary in order that the surface of the base metal be properly deformed and the coating metal welded thereon, that the coating metal, which is in the form of a thin foil, be spaced somewhat from the base metal so that the explosive force can act through such distance as will enable it to accomplish its purpose. It has been found convenient toplace between the surface of the base metal and the coating metal foil a base of wire mesh such as ordinary copper window screening or the like which provides such spacing and other unique properties. The mesh size of the screening may, of course, be varied to suit any particular purpose within limits. In operation, the coating melts and extrudes between the screen interstices to provide a uniquely roughened surface having reentrant portions which serve to anchor in place the coating metal as well as any further metal placed thereon such as bearing metal and the like.

A particular advantage of this invention is that relatively large structures such as bearing sleeves can be tinned in successive conveniently-sized sections. This is as opposed to tinning baths where either a bath large enough to take the largest piece to be treated must be maintained or a number of such baths kept in operable condition.

The following illustrates a typical mode of carrying out the invention.

A steel bearing shell partially represented in FIGURE 1 by 1 was thoroughly cleaned in the area to be tinned. Typically, such surface is cleaned by wetting with a suitable solvent, such as methyl ethyl ketone, and brushing thoroughly with a wire brush. Next, metal Window screening or mesh of the ordinary kind such as copper, aluminum or iron was bent to the approximate curvature of the surface of the shell, placed over the cleaned area and taped in place. The coating metal, in this particular instance tin foil, was placed over the screening and taped along the edges to hold it in place. Next, the sheet explosive 2 cut to proper size was placed over the tin foil and again taped along the edges as at 3 in FIGURE 1. The end of the explosive sheet is preferably allowed to extend somewhat over the edge of the bearing shell as illustrated to permit placement thereon and fixing in place of a blasting cap 4 of the kind used to detonate such materials. Typically, a Hercules J12 engineering cap is used, but it will be realized that any suitable equivalent can be substituted. The blasting cap is connected to a detonating generator, not shown, by means of wires 5. The structure before detonation of the explosive is best shown in partial cross section in FIGURE 2 wherein 6 is the surface of the bearing shell 1, 7 is the juxtaposed screen or mesh, 8 is the layer of tin or other coating metal foil, and 2 is the layer of detonating explosive. Preferably, those parts of the bearing shell which are not being coated are protected as by a rubber or plastic sheet or other protective material. With the coating structure in place, the blasting cap 4 and the explosive sheet 2 are detonated, the foil 8 being melted by the energy of the detonation and forced or extruded between the interstices of the copper screen 7 and coated on the surface 6 of bearing shell 1. While it is not known exactly how the coating takes place, it is suflicient for the purpose of the invention that it does, in fact, take place. It is believed that the force of the detonating explosive 2 deforms the surface of the bearing shell between the interstices of the screen causing a lateral and upward flow of the molten metal under the screen or mesh wires producing a deformed coated surface with reentrant portions similar to dovetailing, the screen itself being forced from the surface so that it is easily removed. The foil 8 is blasted against the deformed surface and molten and welded to the base metal by the heat and energy of the blast to form a continuous conforming coating 10 as in FIGURE 3 having the same rough surface as the surface 9 of the base metal, including reentrant portions such as at 11 forming a tightly olingable layer which not only adheres tenaciously without fluxing to the base metal but forms a good adherent base for any molten bearing metal or other metal which is placed thereon. With a particular portion of the bearing shell so tinned, the procedure is repeated about the inner circumference of the shell, each successive coating being overlapped slightly with that preceding or abutting. With the entire bearing surface tinned, Babbitt metal 12 or other bearing metal is applied to the surface as by centrifugal casting or any other convenient method to form the final bearing shown in partial cross section in FIGURE 4.

The efficacy of the present invention is illustrated by the fact that when metal such as Babbitt metal is applied to such surfaces as by centrifugal casting, the bond strength between the tin and the Babbitt metal is equal to or greater than that attained when the tinned surface is formed merely by machine-roughening the surface and dipping in molten tin with fluxing. Such bond strength is conveniently measured by the so-called Chalmers method in which a test plug of the base metal with its tinned surface and overlying Babbitt is removed, the diameter of the plug being 0.350 inch in diameter. A hole 0.250 inch in diameter is next drilled through the base metal and into the Babbitt metal past the bond line between the Babbitt metal and tin to receive a push rod which is then used to exert force on the Babbitt metal and through it on the annular bonding area between the Babbitt metal and the tin surface bounded by the 0.35 inch plug periphery and the 0.25 inch hole, the point of separation expressed in p.s.i. between the Babbitt metal and the tin being taken as the bond strength.

Generally speaking, it has been found that best results are obtained when the tin foil has a thickness ranging from about 0.010 inch to 0.025 inch and preferably from 0.015 inch to about 0.020 inch. If the tin foil layer is less than about 0.010 inch, it has been found that p or bond strength between the base material and the Babbitt or other overlying metal results. Likewise, poor bond strengths result when the layer is over about 0.025 inch thick. It has been found that the stand-off distance afforded by the wire screen may range from about 0.015 to about 0.045 inch with a preferred stand-off distance or thickness of wire screen being from about 0.030 inch to 0.045 inch. Various mesh sizes of wire screening can be used depending upon the particular application, the preferred sizes ranging from about 20 meshes per inch to about inch mesh. Generally speaking, a detonating explosive charge density of about 1 gram per square inch has been found preferable although the invention can be carried out with charges ranging from about /2 to about 1 /2 grams per square inch.

The following specific examples are illustrative of the present invention and are not intended to be limiting in any way. In each case the method described above Was followed using the thickness of tin indicated in the table below, the screen thickness and mesh set forth and the explosive charge indicated. Structures so prepared were tested for bond strength according to the Chalmers test described above with the results indicated in the table.

Screen Thick- Detonating Chalmers Ex. Thickness of ness (in.) Explosive Bond Tin tin.) and Mesh Charge, Strength (wires/in.) grams/in. (p.s.i.)

0. 0156 0. 030/20 1 13, 040 0. 0156 0. 030/20 1 13, 300 0. 0156 0. 030/20 1 12, 950 0. 0156 0. 030/20 1 13, 380 0. 0156 0. 030/20 1 13, 830 O. 020 0. 045/5 1. 5 9, 610 0. 020 0. 045/5 1. 5 14, 940 0. 020 0. 045/5 1. 5 14, 530 0. 020 0. 045/5 1. 5 10, 210 0. 020 0. 045/5 1. 5 13, 370 0. 020 0. 045/5 1. 5 16, 220 0. 020 0. 045/5 1. 5 12, 720 0. 020 0. 045/5 1. 5 10, 360 0. 020 0. 045/5 1. 5 12, 460 0. 020 0. 045/5 1. 5 15, 570 0. 0156 0 030/20 1. 5 16, 500 0.0156 0 030/20 1. 5 14, 700 0. 0156 O 030/20 1. 5 18, 450 0. 0156 0 030/20 1. 5 14, 260 0. 0156 0. 030/20 1. 5 18, 760 0. 0156 0. 045/5 1. 5 14, 0. 0156 0. 045/5 1. 5 16, 460 0. 0156 O 045/5 1. 5 13, 600 0.0156 0 045/5 1. 5 12, 450

The present invention is also particularly useful in repairing bearings which have become damaged or must be replaced in the field. Instead of removing such bearings or returning them to the factory, the bearing shell may be retinned stepwise as described above, the rest of the equipment being easily protected from the explosive force which, as pointed out above, has the high velocity and high pressure and low energy as opposed to low velocity, low pressure and high, generally destructive energy experienced with deflagrating explosives.

There is provided, then, by the present invention a method of coating base metals with tightly bonded coating metals which serve as an anchoring base for other metals such as bearing metals. While the invention has been particularly described in this connection, it will be realized that it is also useful for providing such coatings in general and for readily preparing metal laminates which are characterized by good bonding between the base metal and overlying metals.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method for coating a 'base metal with a relatively thin layer of coating metal having a relatively low melting point and low heat of fusion which comprises placing a layer of the coating metal in closely spaced relationship to the surface of the base metal to be coated and separated therefrom by a metal screen, placing a layer of detonating explosive over said coating metal layer and detonating said explosive layer whereby said coating metal is melted, flows between the interstices of said screen and bonds to said base metal.

2. The method of claim 1 wherein the layer of coating metal is from about 0.010 to about 0.025 inch thick and said screen is from about 0.015 to about 0.045 inch thick.

3. The method of claim 1 wherein the detonating explosives comprises PETN.

4. The method of claim 1 wherein said coating metal is tin.

5. The method of claim 1 wherein said layer of detonating explosive has essentially the same area as said layer of coating metal.

6. A method for making a clad metal structure which comprises placing a first layer of coating metal having a relatively low melting point and low heat of fusion in closely spaced relationship to the surface of the base metal to be coated and separated therefrom by a metal screen, placing a layer of detonating explosive over said coating metal layer, detonating said explosive layer whereby said coating metal is melted, flows between the interstices of said screen and bonds to said base, removing said screen, and emplacing a second layer of metal on said first coating metal layer and bonding said second metal layer to said first coating metal layer.

7. A method as in claim 6 wherein said coating metal is tin.

8. A method as in claim 6 wherein said second layer is Babbitt metal.

References Cited UNITED STATES PATENTS 3,314,139 4/ 1967 Whittaker et al 29-470.1 3,331,121 7/1967 De Maris et all. 3,346,946 10/ 1967 Riegelmayer 29421 X 3,364,562 1/ 1968 Armstrong 29470.1

JOHN F. CAMPBELL, Primary Examiner.

PAUL M. COHEN, Assistant Examiner. 

1. A METHOD FOR COATING A BASE METAL WITH A RELATIVELY THIN LAYER OF COATING METAL HAVING A RELATIVELY LOW MELTING POINT AND LOW HEAT OF FUSION WHICH COMPRISES PLACING A LAYER OF THE COATING METAL IN CLOSELY SPACED RELATIONSHIP TO THE SURFACE OF THE BASE METAL TO BE COATED AND SEPARATED THEREFROM BY A METAL SCREEN, PLACING A 